The liver fulfils a great variety of essential functions in the body, including the synthesis of proteins involved in metabolism, hemostasis, and protection against infection. Many acquired, complex and genetic diseases (hepatic diseases sensu stricto as well as some hereditary disorders that do not directly lead to liver disease but manifest themselves primarily elsewhere in the body) are associated with altered gene expression in the liver. Some examples include hemophilia A or B, familial hypercholesterolemia, ornithine transcarbamylase deficiency, or α-antitrypsin deficiency. In addition, the liver often falls prey to infections with pathogens (such as hepatitis viruses). Finally, the liver can undergo malignant transformation and give rise to liver cancer (hepatocellular carcinoma) or functionally degenerate as a consequence of pharmaceutical treatments and chemotherapy, drug or alcohol abuse. Consequently, there has been substantial and increasing interest in the use of gene therapy to express a functional gene in the liver to replace a needed protein or to block the expression of an altered or undesired gene product, for instance by RNA interference or dominant-negative inhibitory proteins, or to restore hepatocyte function in a degenerating liver. Transduction of hepatic cells with appropriate genes, such as immunostimulatory cytokines, may also be useful to induce immune responses against e.g. viral hepatitis or liver neoplasms (Barajas et al., 2001; Villa et al., 2001).
One of the major challenges in liver gene therapy is the achievement of hepato-specific therapeutic gene expression (Xia et al., 2004; Prieto et al., 2003). In vivo targeting of mammalian hepatocytes has been done by injecting DNA or viral vectors into the liver parenchyma, hepatic artery, or portal vein. Adenoviral vectors, even when administered systemically, target mainly the liver in mice (Wood et al., 1999) but can also infect lung and skeletal muscle. Moreover, the liver specificity of adenovirus has not yet been demonstrated in humans. Other vectors, like adeno-associated viral vectors (AAV) or lentiviral vectors, can also transduce hepatocytes, but again transduction of non-hepatic cells can occur leading to off-target gene expression (VandenDriessche et al., 2002). Another method to localize gene expression is by transcriptional targeting. In general, transcriptional targeting is highly desirable for all in vivo gene therapy applications as it can prevent expression of the transgene in non-target cells, thus mimicking physiological regulation (Tenenbaum et al., 2003; Schagen et al., 2004). The use of proper liver-specific transcriptional elements should restrict the expression of a therapeutic gene to hepatocytes. For instance, some promoters that are active mainly in the liver have already been used for cell-specific gene delivery (Kuriyama et al., 1991; Kistner et al., 1996). However, functional tissue specificity has only rarely been demonstrated. Furthermore, major disadvantages for the use of liver-specific promoters in gene therapy are the large size, since many vectors have a restricted cloning space, and/or the low activity compared to strong (viral) promoters, such as cytomegalovirus (CMV) or long terminal repeat (LTR) promoter sequences, widely used in gene therapy protocols.
Increasing tissue-specific transgene expression is desirable as a way to decrease the amount of viral vector required to achieve a clinical effect. To increase both specificity and activity, the use of cis-acting regulatory elements has been proposed. Typically, this concerns enhancer sequences, i.e. nucleic acid sequences that increase a promoter's activity and that have the potential to act in cis, and regardless of their orientation, even over relatively long distances (up to several kilobases away from the target promoter). However, enhancer function is not necessarily restricted to such long distances as they may also function in close proximity to a given promoter. For the liver, numerous approaches to incorporate such organ-specific regulatory sequences into retroviral, lentiviral, adenoviral and adeno-associated viral vectors or non-viral vectors (often in addition to house-keeping hepatocyte-specific cellular promoters) have been reported so far (Ferry et al., 1998; Ghosh et al., 2000; Miao et al., 2000; Follenzi et al., 2002). Advantages of restricting vector-mediated gene expression to hepatocytes by using liver-specific promoters and enhancers include e.g., reducing the probability of inducing an immune response to the protein encoded by the transgene (Pastore et al., 1999; Brown et al., 2006, 2007).
Several enhancer sequences for liver-specific genes have been documented. WO95/011308 describes a gene therapy vector comprising a hepatocyte-specific control region (HCR) enhancer linked to a promoter and a transgene. The human apolipoprotein E-Hepatocyte Control Region (ApoE-HCR) is a locus control region (LCR) for liver-specific expression of the apolipoprotein E (ApoE) gene. The ApoE-HCR is located in the ApoE/CI/CII locus, has a total length of 771 bp and is important in expression of the genes ApoE and ApoC-I in the liver (Simonet et al., 1993). In WO01/098482, the combination of this specific ApoE enhancer sequence or a truncated version thereof with hepatic promoters is suggested. It was shown that vector constructs combining the (non-truncated) ApoE-HCR enhancer with a human α-antitrypsin (AAT) promoter were able to produce the highest level of therapeutic protein in vivo (Miao et al., 2000) and may confer sustained expression when used in conjunction with a heterologous transgene (Miao et al., 2001). Of note, these authors not only demonstrate the importance of cis sequences for enhancing in vivo hepatic gene expression, but also reemphasize the lack of correlation of gene expression in tissue culture and in vivo studies.
This ApoE-HCR-AAT expression cassette as used e.g. in the pAAV-ApoHCR-AAT-FIXIA construct (VandenDriessche et al., 2007) is one of the most potent liver-specific FIX expression constructs known, and has been successfully applied in a phase ½ dose-escalation clinical study in humans with severe hemophilia B (Manno et al., 2006). The expression of this hFIX minigene is driven from an ApoE-HCR joined to the human AAT promoter. The 5′-flanking sequence of the human AAT gene contains multiple cis-regulatory elements, including a distal enhancer and proximal sequences, with a total length of around 1.2 kb. It was shown to be sufficient to confer tissue specificity in vivo by driving gene expression primarily in the liver and also, to a lesser extent, in other tissues known to express AAT (Shen et al., 1989). A 347 bp fragment of this 1.2 kb region in combination with the ApoE enhancer is capable of achieving long-term liver-specific gene expression in vivo (Le et al., 1997). Interestingly, this shorter promoter targets expression to the liver with a greater specificity than that reported for larger AAT promoter fragments (Yull et al., 1995).
Other chimeric liver-specific constructs have also been proposed in the literature, e.g. with the AAT promoter and the albumin or hepatitis B enhancers (Kramer et al., 2003), or the alcohol dehydrogenase 6 (ADH6) basal promoter linked to two tandem copies of the apolipoprotein E enhancer element (Gehrke et al., 2003). The authors of the latter publication stress the importance of the relatively small size (1068 bp) of this enhancer-promoter combination.
To be able to provide a therapeutic level of the transgene product for an extended time period, gene transfer vectors preferably allow specifically regulated, high expression, while at the same time retaining sufficient cloning space for the transgene to be inserted, i.e. the regulatory elements used to achieve the high and tissue-specific expression preferably are of only limited length. However, none of the gene therapy vectors disclosed thus far satisfies all these criteria. Instead, gene therapy vectors are not sufficiently robust in terms of either expression levels and/or specificity of expression in the desired target cells, particularly the hepatocyte. Decreasing the promoter/enhancer size often compromised the expression levels and/or expression specificity whereas the use of larger sequences often compromises the efficiency of gene delivery due to impaired vector function, packaging and/or transfection/transduction efficiency. Thus, there is a need in the art for vectors that achieve therapeutic levels of transgene expression in the liver for effective gene therapy.